Field of the Invention
The present invention relates to a brushless motor, and, more particularly, to a flat brushless motor which has a plurality of spiral coils fixed to a stator yoke, a permanent magnet fixed to a rotor at a position at which it faces the spiral coils, and a sensor element for detecting the rotational phase of the rotor so that current can be supplied to the coils sequentially while being switched over at predetermined timings, to rotate the motor.
Description of the Related Art
(i) Brushless motor employing Hall element
FIG. 1 shows a known capstan motor for use in a magnetic recording/reproducing device, as an example of a flat brushless motor of the above-described type. The motor has a stator yoke 1 fixed to a housing 2, a plurality of coils 3 disposed around the circumference of the stator yoke 1, a rotor yoke 4 supported on a rotary shaft 5 by a bush 6, and a multipole magnet 7 mounted on the rotor yoke 4 in such a manner that it faces the spiral coils 3. The motor further includes an FG magnet 8 which is mounted around the periphery of the rotor yoke 4 and consists of magnets disposed at a small pitch, and a magnetic detecting element 9 mounted on the stator yoke 1 which detects variations in the magnetic field generated by the magnet 8 so as to detect the rotation of the rotary shaft 5. A semiconductor magnetoresistive element which detects variations in magnetic reluctance is generally used as the magnetic detecting element 9.
A Hall element 10 is fixed to the stator yoke 1 to detect the phase of the multipole magnet 7 when the rotor yoke 4 is rotated about the shaft 5.
The housing 2 supports the rotary shaft 5 by ball bearings 11 and a metal bearing 12.
In the thus-arranged brushless motor, current is supplied to the coils 3 and is switched at predetermined timings in response to the output of the Hall element 10, to generate a rotational torque and thereby rotate the rotor yoke 4. Assuming that this motor has three phases, it generates a composite torque in the form shown in FIG. 2.
More specifically, if the number of poles of the multipole magnet 7 is n, the coils 3 are disposed on the stator yoke 1 in a state wherein they are separated from one another with respect to the poles of the magnet 7 through a phase angle of ##EQU1## an electrical angle of 120.degree.. Therefore, the distribution of magnetic flux density in the coils 3 varies in the form of a sine wave wherein the sine waves of the first phase, second phase, and third phase are shifted in the manner shown in FIGS. 2 (A), (B), and (C), respectively. Positive and negative currents are sequentially supplied to the coils 3 in response to the output of the Hall element 10 at timings shown in FIGS. 2 (B) (a), (b), and (c), generating torques shown in FIG. 2 (C) (d). These torques are combined to form a wave-shaped torque which has torque ripples, as shown in FIG. 2 (C) (e).
These torque ripples cause irregularities in the rotation. The relationship between the torque ripples and rotation irregularities is expressed by the following Equation: ##EQU2## where T is the magnitude of the torque ripples; N, the rotational speed of the rotary shaft 5; J, its moment of inertia; and .DELTA.N, the magnitude of the rotation irregularities. Accordingly, if a capstan motor has a low rotational speed N, the torque ripples T tend to increase the magnitude of the rotation irregularities .DELTA.N, causing wow and flutter in low-frequency audio signals or jitter in video signals in a magnetic recording/reproducing device. Conventionally, these problems have been dealt with by increasing the moment of inertia. However, this method runs counter to the tendency of decreasing the weight and size of the entire device.
(ii) Brushless motor with magnetoresistive element
A brushless motor employing a magnetoresistive element is generally constructed as shown in FIG. 3. It has a rotor with a signal magnet 14 which is mounted around the periphery thereof and consists of a magnet having a plurality of poles, a main magnet 16, a rotary shaft 19, and a yoke 20; and an MR (magnetorestrictive) element 13 fixed at a predetermined position to detect the magnetic field generated by the signal magnet so thus detect the rotational speed.
In such a single-point detection system, when the signal's wave-length is short, variations in the detection level are likely to be affected by errors in mechanical accuracy (such as deflection of the shaft or deflection of the outer peripheral surface). Further, a shorter signal wavelength reduces the intensity of the signal's magnetic field, thereby reducing the output of the MR element. Reductions in the moment of inertia and speed of the rotor, which are the results of recent trends in the reduction in size of the motor, are countered by increasing the FG frequency. In the above-described example, this is attained by increasing the number of poles of the magnet mounted around the outer periphery, i.e., by making the wavelength of the signal magnet shorter. However, there is a limit to the decrease in the wavelength, as well as to the reduction in errors in mechanical accuracy, resulting in the occurrence of variations in the detection level and also measurement errors.
A Hall element is generally used as a rotational position detecting means which picks up leakage flux from the main magnet 16 of FIG. 3 and determines the polarities of the magnetic poles to switch over the current supplied to the coils 17 of the motor. The Hall element is generally disposed in, for example, a hole or notch in the stator yoke 18 to pick up the leakage flux from the main magnet 16. However, the provision of the hole or notch in the stator yoke causes variations in motor torque, causing cogging. It also requires a high degree of mechanical accuracy, since an error in the position of the Hall element will cause erroneous switch-over of the current supplied to the coils 17, generating variations in torque. The provision of a hole or notch in the surface which faces the main magnet 16 causes a degradation in rotation irregularities of the motor. The Hall element could be disposed between the magnet and the stator yoke 18, but in such a case, the size or thickness of the element must be taken into consideration if the motor is small and flat. In addition, the motor shown in FIG. 3 has a three-phase, full wave drive, and therefore requires three Hall elements to switch over the current for the coils. This in turn requires a total of eight wires from the Hall elements, which occupies a large space on the wiring board.